1. Field of the Invention
The present invention relates to a structure including a photonic crystal and a surface-emitting laser including the structure.
2. Description of the Related Art
In Optics Express, vol. 13, No. 17, 6564 (2005), guided-mode resonance in a photonic crystal illustrated in FIG. 8 is discussed.
As shown in FIG. 8, a photonic crystal layer 8050 is disposed on a sapphire substrate 8000 and has a GaN layer 8010, a nucleation layer (AlN) 8030, and holes 8020 periodically arranged in the GaN layer 8010.
“Guided-mode resonance” is a phenomenon in which guided-mode light propagating in a planar direction of a photonic crystal having a photonic periodic structure in the planar direction couples with the radiation mode light and is thereby emitted to the exterior of the photonic crystal.
This guided-mode resonance allows the photonic crystal to be used as a mirror. Light incident perpendicular to the surface of the photonic crystal can be converted to waveguide mode light in the photonic crystal through a coupling with a mode that exists above a light line. Then, this light is emitted to the outside of the photonic crystal through the coupling with the radiation mode. Optical interference between directly reflected light without the coupling with the waveguide mode light and emitted light coupled with waveguide mode light results in specific reflection, i.e., total reflection without loss.
Generally, a “waveguide mode” refers to a condition under which light does not leak out of a photonic crystal layer while a “radiation mode” refers to a condition under which light leaks out of a photonic crystal layer.
The “light line” refers to a dispersion relation of transmission light in a medium adjacent to a waveguide layer (photonic crystal layer). This light line is shown in a linear line satisfying an equation w=ck/n (w: angular frequency, c: light velocity, n: refraction index, k: wave number). Generally, in a higher frequency region than the light line, light leaks easily.
The principle of the function of a mirror using guided-mode resonance is different from that of a mirror using a photonic band gap.
When light incident perpendicular to the photonic crystal layer, as shown in FIG. 8, is adjusted to have a spectrum in which reflectivity is significantly increased by the guided-mode resonance, the incident light is reflected with high reflectivity.
In the above-mentioned reference, changes in the guided-mode resonance are simulated by using as a simulation parameter the refractive index of the sapphire substrate 8000 (reflective index is 1.8), which is adjacent to the photonic crystal layer 8050 (reflective index of GaN is 2.37).
FIG. 9 is a graph showing a profile of the transmission spectrum of the guided-mode resonance. The abscissa denotes frequency and the ordinate denotes transmission rate. The refractive index of the substrate (“n” in the graph) is varied where the refractive index of the photonic crystal layer is constant.
With respect to FIG. 9, when the relative refractive index difference Δn (=(nphc−nclad)/nphc) between the photonic crystal layer (refractive index is nphc) and the substrate (refractive index is nclad), which functions as a cladding layer disposed adjacent to the photonic crystal layer, becomes small, an occurrence of guided-mode resonance is suppressed.
Specifically, the relative refractive index difference between the photonic crystal layer and the substrate is about 0.24 (approximately 24%) where nphc=2.37 and nclad=1.8 (“n” in FIG. 9 corresponds to nclad) . In this case, guided-mode resonance can occur as shown in FIG. 9.
However, when the relative refractive index difference between the photonic crystal layer and the substrate is about 0.10 (approximately 10%) or lower, for example, nphc=2.37 and nclad=2.135, the occurrence of the guided-mode resonance is suppressed.
When the guided-mode resonance in the photonic crystal is applied to a laser cavity and the like as a mirror, it is often necessary to use a structure of a mirror having a very small relative refractive index difference between the photonic crystal layer and the cladding layer that is adjacent to the photonic crystal layer.
For example, air having a low refractive index can be used as a medium adjacent to a photonic crystal layer when a certain relative refractive index difference is desired between the photonic crystal layer and the medium that is adjacent to the photonic crystal layer. This “air gap,” however, is not easy to make using a semiconductor lamination process. Therefore, in an application for optical devices, a structure in which guided-mode resonance can be induced is desired even in the case where a semiconductor material having a higher refractive index than that of air is disposed adjacent to the photonic crystal layer.
When a photonic crystal layer is applied to a cavity of a surface-emitting laser device that emits light having a wavelength of 670 nm as a substitute for a laminated film mirror, Al0.5Ga0.5As (refractive index is 3.446) may be used as the photonic crystal layer and Al0.93Ga0.07As (refractive index is 3.130) may be used as a cladding layer adjacent to the photonic crystal layer.
In this case, the relative refractive index difference Δn(=(nphc−nclad)/nphc) is about 9.2%. Since the structure described in OPTICS EXPRESS, vol. 13, No. 17, 6564 (2005) may not generate guided-mode resonance, it is difficult to use a photonic crystal layer instead of a laminated film mirror.
The present invention provides a structure that can generate guided-mode resonance, and provides a surface-emitting laser using the structure. The structure can generate the guided-mode resonance even if a relative refractive index difference is insufficient, for example, as in the case of using a GaN layer 8010 as the photonic crystal layer and the sapphire substrate 8000 as the layer adjacent to the GaN layer 8010.